Polyethylene (PE) is synthesized via polymerising ethylene (CH2=CH2) monomer and optionally a higher 1-olefin comonomer such as 1-butene, 1-hexene, 1-octene or 1-decene. Because PE is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the synthesis methods, PE can be generally classified into several types such as LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
It is known that the polymerisation of olefins e.g. ethylene, especially by a gas phase polymerisation process, involves the polymerisation of olefin monomer with the aid of catalyst and optionally, if required depending on the used catalyst, a co-catalyst. Suitable catalysts for use in the production of polyolefins, and in particular for the preparation of polyethylene, comprise chromium-type catalysts, Ziegler-Natta catalysts and metallocene catalysts.
According to the present description, the term “catalyst” is defined herein as a substance that causes a change in the rate of a polymerisation reaction without itself being consumed in the reaction. Any catalyst allowing ethylene to be polymerised may be used. By way of examples of such catalysts, mention may be made of catalysts of the Ziegler-Natta type, catalysts based on vanadium or chromium, and metallocene catalysts. According to a preferred embodiment said catalyst is a metallocene.
The term “metallocene catalyst” is used to describe any transition metal complexes consisting of metal atoms “sandwiched” between one or two ligands. In a preferred embodiment, the metallocene catalyst has a general formula MX, wherein M is a transition metal compound selected from group IV and wherein X is a ligand composed of one or two groups of cyclopentadienyl (Cp), indenyl, fluorenyl or their derivatives. Illustrative examples of metallocene catalysts comprise but are not limited to Cp2ZrCl2, Cp2TiCl2 or Cp2HfCl2.
The metallocene catalysts generally are provided on a solid support. The support should be an inert solid, which is chemically unreactive with any of the components of the conventional metallocene catalyst. The support is preferably a silica compound.
The use of metallocene catalysts in the production of polyolefins in general, and of polyethylene in particular, is known in the art. The metallocene catalysts are compounds of Group IV transition metals of the Periodic Table such as titanium, zirconium, hafnium, etc., and have a coordinated structure with a metal compound and ligands composed of one or two groups of cyclopentadienyl, indenyl, fluorenyl or their derivatives. Use of metallocene catalysts in the polymerisation of olefins has various advantages. Metallocene catalysts have high activities and are capable of preparing polymers with enhanced physical properties in comparison with the polymers prepared using Ziegler-Natta catalysts. Metallocene catalysts are usually employed with a co-catalyst such as an organometallic compound, or a mixture of non-coordinated Lewis acid and alkylaluminium as it is well known in the art. The key to metallocenes is the structure of the complex. The structure and geometry of the metallocene can be varied to adapt to the specific need of the producer depending on the desired polymer. Metallocenes comprise a single metal site, which allows for more control of branching and molecular weight distribution of the polymer. Monomers are inserted between the metal and growing chain of polymer.
As used herein, the term “catalyst slurry” refers to a composition comprising catalyst solid particles that are in suspension. The term “concentrated catalyst slurry” refers to a composition comprising catalyst solid particles that are in suspension whereby the concentration of catalyst is at least higher than 10% by weight. The term “diluted catalyst slurry” refers to a composition comprising catalyst solid particles that are in suspension, whereby the concentration of catalyst is lower than or equal to 10% by weight. The diluent is typically a hydrocarbon diluent.
It is well known that the polymerisation reaction is quite sensitive to the quantity of catalyst utilized. It is important to control catalyst flow to a reactor since unexpected or uncontrolled catalyst injection in a reactor could lead to runaway reactions. Furthermore, metallocene catalysts are usually employed with a co-catalyst for olefin polymerisation, which can significantly enhance the polymerisation efficiencies to beyond a million units of polymer per unit of catalyst. A solution to problems due to these facts has been proposed in the patent application WO 2005/077522.
At the moment, the catalyst productivity is estimated by first measuring the amount of silica in the final PE powder by X-ray fluorescence (XRF). The productivity of the catalyst is then calculated as g PE/g catalyst that has entered the reactor. The problem with this current method is that metallocene catalysts have a high productivity with relatively low quantities of silica, making it difficult to measure the amount of silica in the final PE leading to important errors in the calculation and thus in the estimated productivity. The estimated productivity can also not be controlled in a rapid manner that is sometimes required for on-line control of the process.
It is therefore a general object of this invention to provide a method and system for measuring catalyst productivity in an accurate manner, especially in the case of metallocenes. It is a further object of the present invention to provide a method and system for measuring catalyst productivity on-line and/or without any significant delays. Furthermore, the present invention aims to provide an improvement to the system and method described in WO 2005/077522.